APS are solid state imagers where each pixel contains a photo-sensing means, charge to voltage conversion means, reset means, and all or part of an amplifier. They have some advantages over charge coupled device imagers (CCD), including single 5 V supply operation, x-y addressability, and high level of integration of on-chip signal processing.
APS devices are operated in a manner where each pixel is repeatedly reset and read out. The reset operation is accomplished by resetting the photodetector or charge to voltage conversion means by removing the signal charge that is residing there. This is accomplished by incorporation of a reset transistor in each pixel. Turning the reset transistor on allows signal charge to flow into the drain of the reset transistor, thus being removed from the photodetector region or charge to voltage conversion region. Since a reset transistor is included in each pixel, it occupies area that could otherwise be used for the photodetector, thus reducing the fill factor and sensitivity of the device.
A prior APS pixel is shown in FIG. 1. The pixel 10 comprises a photodetector 12, which can conceivably be either a photodiode or photogate, transfer gate 14, floating diffusion 16, reset transistor 17, with a reset gate 18 and reset drain 19, row select transistor (ROWST) 8, with a row select gate (RSG) 9 and signal transistor (SIG) 6. Pixels are arranged in an array (X-columns and Y-rows), to form an image sensor. Device operation occurs in the following manner. A given row is reset by turning on transfer gate 14 and reset gate 18. Any electrons that are in the photodetector 12 or floating diffusion 16 are removed via the reset drain 19. The transfer gate 14 and the reset gate 18 are then turned off and incident light creates electrons in the photodetector 12 for a determined time (integration time). The reset gate is then turned on, removing any electrons that may have accumulated in the floating diffusion region. The reset gate is then turned off and the reset signal level is then read out one column at a time for that row (the details of this operation are not relevant to this invention). Transfer gate 14 is then turned on and these electrons are transferred onto the floating diffusion 16 which is connected to the gate of SIG 6. This signal level is then read out a column at a time for that particular row. A CDS amplifier in each column is used to remove reset noise and noise due to pixel offset. This operation is then repeated for the remaining rows, with the integration time being of constant duration for each row, but integrated during a different time period. As discussed earlier, the incorporation of a reset transistor in each pixel reduces the fill factor and sensitivity of the device. Additionally, the row by row reset integrate and read operation can produce image artifacts.
There are two basic types of charge to voltage conversion means, the floating diffusion and the floating gate. The floating diffusion method provides very good sensitivity, (i.e. small capacitance), but results in an incomplete reset which leads to reset noise. The FG method provides a complete reset and consequently no reset noise, but has poor sensitivity. The floating diffusion approach has been used predominantly in APS and charge coupled devices, (CCD) to obtain good sensitivity. The reset noise is dealt with by performing correlated double sampling (CDS).
In order to perform CDS and eliminate the reset noise the reset operation must occur prior to the read operation. Because of this need to have a reset signal level prior to a read signal level, APS devices have been operated in a manner where each line or row of the imager is reset, integrated and read out at a different time interval than each of the remaining lines or rows. Hence if one were reading out the entire imager, each line would have captured the scene at a different point in time. Since illumination conditions can and do vary temporally, and since objects in the scene may also be moving, this method of read out can produce line artifacts in the resulting representation of the image. This limits the usefulness of APS devices in applications where high quality motion or still images are required. This problem can be overcome by reading the signal level then the reset level, but as stated above this would not eliminate the reset noise.
In order to solve the problems of APS devices described above, it is desirable to provide a reset means that does not reduce the fill factor of the pixel. It is also desirable to provide a reset means that is complete so that the APS device could be operated in a manner where it is read then reset without introduction of reset noise.